1. Technical Field
The present invention relates to methods of manufacturing wavelength converters, and to wavelength converters themselves.
2. Description of the Related Art
Semiconductor lasers and solid-state lasers possess in their constitutional material unique output wavelengths, and the extent of the range of the wavelengths directly ties into the extent of the lasers' fields of application. Infrared light sources are employed in environmental sensing and medical/biotech applications, while application to fields including vehicle emissions testing, laser ionization mass spectrometry, analysis of fruit sugar content, dental care, noninvasive blood inspection, and cerebral blood flow measurement are under investigation.
Nevertheless, light sources such as ruby lasers, yttrium aluminum garnet (YAG) lasers, and carbon dioxide gas lasers, for example, can emit only light of a given wavelength. And while a light source such as the titanium-sapphire laser is tunable, it can emit only light of wavelengths near 650 nm to 1100 nm. Thus it is not possible to obtain laser light across the entire wavelength range. Accordingly, wavelength converters that can convert light of a given of wavelength emitted from a laser light source into light of a different wavelength are being sought.
Wavelength converters employing borate crystals such as barium borate (BBO) or lithium borate (LBO) have been widely known to date. In this type of wavelength converter, wavelength conversion takes place by means of phase matching exploiting the birefringence of the crystal. With wavelength converters exploiting crystal birefringence, however, obtaining adequate wavelength-conversion efficiency is problematic. And inasmuch as the birefringence, being an inherent property of the crystal, cannot be adjusted, the degrees of freedom, including wavelength selectivity, of wavelength converters exploiting birefringence are low.
In that regard, Japanese Unexamined Pat. App. Pub. No. 2008-170710 (Patent Document 1) discloses a wavelength converter employing a compound semiconductor crystal that contains nitrogen (N) and at least gallium (Ga) or aluminum (Al) or indium (In), and that exhibits spontaneous polarization. In Patent Document 1, a poled structure in which the spontaneous polarization is periodically reversed in a two-dimensional lattice geometry is formed in the compound semiconductor crystal, with the poled structure satisfying quasi-phase-matching (QPM) conditions two-dimensionally for an incoming beam of a first wavelength. Long interaction lengths, compared with conversion exploiting borate-crystal birefringent matching, are therefore yielded, enabling highly efficient wavelength conversion.
The foregoing Patent Document 1 further discloses a method of manufacturing a wavelength converter, with a two-dimensional domain inversion structure being formed using a compound semiconductor crystal. In particular, a mask patterned to correspond to the pattern of the two-dimensional domain inversion structure is formed onto a gallium nitride (GaN) substrate having a +c-plane. A +c-axis oriented GaN layer is thereafter formed onto the patterned mask and the +c-plane of the GaN substrate. In this case, onto the +c-plane of the GaN substrate, a +c region is epitaxially grown in such a way that the thickness of the GaN layer increases in the +c-axis direction, while onto the mask layer, a −c region is epitaxially grown in such a way that the thickness of the GaN layer increases in the −c-axis direction. A two-dimensional domain inversion structure is thereby formed.    Patent Document 1: Japanese Unexamined Pat. App. Pub. No. 2008-170710